tests: centralize default CPU arguments in test infrastructure

Replace the hard-coded  .args(["--cpus", "boot=1"]) in the simple
launch integration test with a shared helper (default_cpus) from test
infrastructure.

Extend Guest with explicit CPU-related defaults (num_cpu, nested)
and add default_cpus_string() so CPU configuration is derived from
guest state instead of being duplicated at call sites.

This refactor improves consistency and makes CPU defaults easier to
maintain across integration tests.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

cloud-hypervisor/tests/integration.rs

@@ -2545,7 +2545,7 @@ fn _test_simple_launch(guest: &Guest) {
     let event_path = temp_event_monitor_path(&guest.tmp_dir);
 
     let mut child = GuestCommand::new(guest)
-        .args(["--cpus", "boot=1"])
+        .default_cpus()
         .args(["--memory", "size=512M"])
         .default_kernel_cmdline()
         .default_disks()